fix pipeline

installing python packages without gcc seems to be very hard
also fix pip caching

.gitlab-ci.yml

@@ -5,12 +5,13 @@ variables:
 
 cache:
   paths:
-    - "$CI_PROJECT_DIR/pip-cache"
+    - "$PIP_CACHE_DIR"
   key: "$CI_PROJECT_ID"
 
 .install_sphinx:
   before_script:
-  - pip install -U -r docs/requirements.txt
+  - apk add gcc musl-dev
+  - pip install --cache-dir "$PIP_CACHE_DIR" -U -r docs/requirements.txt
 
 test-docs:
   stage: test